Interspinous spacer devices for dynamic stabilization of degraded spinal segments

ABSTRACT

An interspinous spacer device for the treatment of high-grade spinal disorders is disclosed herein. The interspinous spacer device includes a sliding rod and a base that contains a curved internal track that limits the range of motion and center of rotation of the spinal segments stabilized using the device to the physiological levels of a nondegraded spinal segment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/983,036 filed Jul. 31, 2013, which application is a national stageentry of PCT/US2011/050370 filed Sep. 2, 2011, which claims the benefitunder 35 U.S.C. §119(e) of: U.S. Provisional Application 61/438,719,filed Feb. 2, 2011. The contents of the above-mentioned patentapplications are hereby incorporated by reference in their entireties.

The present application is also related to pending U.S. patentapplication Ser. No. 13/983,020, which is entitled “Pedicle ScrewAssembly and Dynamic Stabilization Devices Incorporating the PedicleScrew Assembly”, filed Jul. 31, 2013, and incorporated by reference inits entirety into the present application.

FIELD OF THE INVENTION

The present invention relates to devices for the treatment of high-gradespinal disorders. More specifically, the present invention relates tointerspinous spacer devices.

BACKGROUND OF THE INVENTION

Posterolateral fusion is the standard procedure for treating high gradespinal disorders such as spinal stenosis as well as spondylolisthesis.Despite the wide-spread use of posterolateral fusion as a surgicalapproach for correcting back pain, numerous problems have beenassociated with its use. Spinal fusion recipients may be at risk fordeveloping Adjacent Segment Disease (ASD), a condition in which themotion segments adjacent to the fused vertebral segments experiencehigher rates of degeneration deterioration due to an increase invertebral loading, higher intradiscal pressures, increased range ofmotion, and increased facet motion.

Dynamic spinal stabilization has recently emerged as an alternativeprocedure to treat many degenerative spinal disorders. Existing dynamicstabilization devices restore stability to an injured spine whilesimultaneously allowing a restricted range of motion. These devices aredesigned to preserve the integrity of adjacent segments by minimizingthe transfer of segment motion and facet joint forces between thestabilized spinal segment and the adjacent spinal segments.

Existing dynamic spine stabilization devices incorporate selectivelyflexible elements such as flexible cords and intervertebral spacers, orflexible spring rods in order to allow a constrained range of motion tothe stabilized spinal segment. To date, no existing dynamic spinestabilization device constrains the rotation of the stabilized segmentsto a center of rotation that is coincident with a physiological centerof rotation. Physiologically representative loading of a spinal segmentthat is stabilized using a dynamic stabilization device is unlikely tooccur unless the rotational motion of the spinal segment passes throughthe spine's natural center of rotation. The imposition of anon-physiological center of rotation location by existing dynamicstabilization devices may result in alterations to the physiologicalpattern of tissue stresses and may further increase the likelihood ofhardware failure. These altered tissue stresses and non-physiologicalmotion patterns may also be induced in adjacent motion segments,increasing the likelihood of long-term complications, such as ASD,associated with existing stabilization procedures.

In addition, at least some of the existing dynamic spine stabilizationdevices incorporate pedicle screws in their design. However, thetreatment of back pain using pedicle-based implants may pose anincreased risk of complications in certain patient populations. Toaddress potential risks of the treatment of back pain using pediclescrew-based stabilization devices, interspinous spacer devices may beused to correct spinal stenosis and facet arthrosis when a less invasivesurgical procedure is preferred or when pedicle screw use isunsuccessful.

Interspinous spacers are an appropriate treatment for patientsexperiencing neurogenic pain that is relieved in flexion and exacerbatedin extension. Existing interspinous spacer designs aim to unload theintervertebral disc and increase the neuroforaminal height by limitingthe amount of motion available during extension. These existinginterspinous spacers typically focus closely on the stabilization ofextension movements while neglecting lateral bending, axial rotation,and sometimes even flexion movements. As a result, many existinginterspinous spacer devices provide limited stability in lateral bendingand axial rotation. A variety of attachment methods are used forexisting interspinous spacer devices including polyester tethers andmetal clamps, and it is unclear how these fasteners may influence themotion segment's center of rotation during spinal flexion movements. Inaddition, device slippage and spinous process failure are potentialcomplications associated with the use of existing interspinous spacerdevices.

There is a need in the art for an interspinous spacer device that notonly allows limited motion of injured or deteriorated vertebralsegments, but that constrains that motion to a range that is consistentwith the range of motion of the corresponding normal healthy vertebrae.In particular, a need exists for an interspinous spacer device in whichthe degraded vertebrae are constrained to rotate about an axis that isconsistent with a normal healthy spine.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, an interspinous spacer device for the stabilizationof a degraded spinal segment is disclosed herein. The interspinousspacer device includes a base and a sliding rod. The base includes abody containing a curved internal track that opens upward into anaperture contained within an upper surface of the body. The aperture hasan aperture cross-sectional area that is smaller than a correspondingtrack cross-sectional area of the track. The sliding rod includes aretaining plate situated within the track as well as a column attachedto the retaining plate. The retaining plate has a plate cross-sectionalarea that is larger than the aperture cross-sectional area and smallerthan the track cross-sectional area. The column includes an upper columnend and a lower column end opposite to the upper column end. The lowercolumn end is attached to the retaining plate and the upper column endprotrudes upward through the aperture and out of the upper surface ofthe body. The retaining plate of the sliding rod freely slides withinthe track to actuate an arcuate movement of the sliding rod along arange of movement limited by a length of the track. The center ofrotation of the arcuate movement is situated at a perpendicular distancefrom the track that is equal to a radius of curvature of the track.

In another embodiment, an interspinous spacer device for thestabilization of a degraded spinal segment is disclosed herein. Theinterspinous spacer device includes a base and a sliding rod. The baseincludes a body containing a curved internal track that is bounded atopposite ends by a lower track wall and an upper track wall, and boundedlaterally by a curved anterior track wall, a curved posterior trackwall, and two side track walls. The track opens upward into an aperturecontained within the track upper wall that extends through an uppersurface of the body. The aperture has an aperture cross-sectional areathat is smaller than a corresponding track cross-sectional area of thetrack. The sliding rod includes a retaining plate situated within thetrack. The retaining plate includes an upper surface, a lower surface,an anterior face, a posterior face, and two side faces. The retainingplate has a plate cross-sectional area that is larger than the aperturecross-sectional area and smaller than the track cross-sectional area.

In this embodiment, the column includes a lower column end attached tothe upper surface of the retaining plate and an upper column endsituated opposite to the lower column end and protruding upward from theupper surface of the body through the aperture of the body. Theretaining plate of the sliding rod slides freely within the track toactuate an arcuate movement of the sliding rod along a range of movementlimited by the track. The center of rotation of the arcuate movement issituated at a perpendicular distance from the anterior track wall thatis equal to a radius of curvature of the track.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. As will be realized, theinvention is capable of modifications in various aspects, all withoutdeparting from the spirit and scope of the present invention.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic drawings of the load distribution on anormal vertebral segment (FIG. 1A) and on a vertebral segmentconstrained to rotate about a different center of rotation by a dynamicstabilization device.

FIGS. 2A and 2B are drawings illustrating the effect of concave (FIG.2A) versus convex (FIG. 2B) curvature of interspinous spacer devicesurfaces on the location of its center of rotation.

FIG. 3 is a photograph of a prototype interspinous spacer deviceinstalled between the spinous processes of two lumbar vertebrae.

FIG. 4 is a side sectional view of an interspinous spacer device.

FIG. 5 is a drawing showing the arcuate movement of a sliding rod withina base of an interspinous spacer device and the resulting center ofrotation of the movement.

FIG. 6 is a perspective view of a sliding rod of an interspinous spacerdevice.

FIG. 7 is a side view of a sliding rod of an interspinous spacer device.

FIG. 8 is a perspective view of a base of an interspinous spacer device.

FIG. 9 is a side view of a base of an interspinous spacer device.

FIG. 10 is an image of a finite element model of an interspinous spacerdevice implanted in a spinal segment.

FIG. 11 is an image of a finite element model of an interspinous spacerdevice implanted in a spinal segment showing the center of rotationduring flexion.

FIGS. 12A-D are graphs showing the relative rotation as a function ofapplied flexion/extension moment estimated using finite element modelsduring spinal flexion and extension for an intact spine, an injuredspine, and an injured spine stabilized with an interspinous spacerdevice implanted at the L2-L3 level for four different lumbar spinesegment levels: L1-L2 (FIG. 12A), L2-L3 (FIG. 12B), L3-L4 (FIG. 12C),and L4-L5 (FIG. 12D).

FIG. 13 is a graph comparing the relative rotation of 4 different spinalsegment levels and the total lumbar rotation estimated using finiteelement models during spinal flexion and extension for an intact spinean injured spine, and an injured spine stabilized with an interspinousspacer device implanted at the L2-L3 level.

FIG. 14 is an image of a finite element model of a sliding rod of aninterspinous spacer device showing the region of maximum stress duringflexion.

FIG. 15 is an image of a finite element model of a base of aninterspinous spacer device showing the region of maximum stress duringflexion.

FIG. 16 is an image of a finite element model of an interspinous spacerdevice with a modified design implanted in a spinal segment.

FIG. 17 are graphs showing the relative rotation as a function ofapplied flexion/extension moment estimated using finite element modelsduring spinal flexion and extension for an intact spine, an injuredspine, and an injured spine stabilized with an interspinous spacerdevice with a modified design implanted at the L2-L3 level for differentlumbar spine segment levels: L1-L2 (FIG. 17A), L2-L3 (FIG. 17B), L3-L4(FIG. 17C), and L4-L5 (FIG. 17D).

FIG. 18 is a graph comparing the relative rotation of 4 different spinalsegment levels and the total rotation estimated using finite elementmodels during spinal flexion and extension for an intact spine, aninjured spine, and an injured spine stabilized with an interspinousspacer device with a modified design implanted at the L2-L3 level.

FIG. 19 is an image of a finite element model of a sliding rod of aninterspinous spacer device with a modified design showing the region ofmaximum stress during flexion.

Corresponding reference characters indicate corresponding elements amongthe views of the drawings. The headings used in the figures should notbe interpreted to limit the scope of the claims.

DETAILED DESCRIPTION

Interspinous spacer devices for the relief of conditions including butnot limited to spinal stenosis and facet arthrosis are disclosed herein.Embodiments of the interspinous spacer devices increase theneuroforaminal height between two vertebrae by limiting the motionsegment's rotation in extension while preserving physiologic loading ofthe vertebrae in flexion by projecting the motion segment's center ofrotation to the segment's natural, undegenerated location. Further, theembodiments of the interspinous spacer devices restore the stresses ofoverloaded facet joints to natural, undegenerated levels. The design ofembodiments of the interspinous spacer devices also increase stabilityin the transverse and coronal planes by secure attachment to the spinousprocesses while limiting the stresses experienced at these interfaces.

a. Principle of Design

Embodiments of the interspinous spacer devices provide enhancedstructural support to compensate for degenerated spinal structures whilesimultaneously preserving a range of motion that is comparable to thenatural motion of the undegenerated spinal segment. A critical factorgoverning the motion and segment loading of a stabilized spinal segmentis the location of the center of rotation of the stabilized segment.

Center of rotation, as used herein, describes the spatial location of anaxis of rotation about which two adjacent vertebrae rotate relative toone another in the course of an overall rotation of the spine. Theoverall rotation of the spine may occur as the result of any number ofmovements including but not limited to dorso-ventral flexion andextension, lateral bending to the left or right, axial rotation(twisting) and any combination thereof. In order to accomplish any ofthese overall movements, individual adjacent vertebrae rotate relativeto one another in a variety of directions. In the process of thesemovements, loads are also transmitted between adjacent vertebrae in acharacteristic pattern.

FIG. 1A is a schematic diagram illustrating the center of rotation andload distribution between two adjacent vertebrae of a healthy spineduring an overall posterior extension movement. Typically, the center ofrotation is located somewhere within the anterior portion of the spineas shown in FIG. 1A. This center of rotation further results in tensileloading in those regions of the vertebra anterior to the center ofrotation and compressive loads in those regions posterior to the centerof rotation.

Existing interspinous spacer devices may include a hinge or otherrotating element in the region posterior to the vertebral disks due tothe constraints imposed by the task of implanting structurallyreinforcing devices onto posterior spinal structures using existingsurgical procedures. As a result, the center of rotation of a spinalsegment stabilized using existing interspinous spacer devices may havecenters of rotation that are shifted significantly in a posteriordirection, as illustrated in FIG. 1B. In addition, the loading patternbetween the two adjacent vertebrae may be altered such the vertebraexperience tensile loading over a significantly larger proportion oftheir anterior regions, and significantly less compressive loadingduring the posterior extension movement illustrated in FIG. 1B.

The instantaneous center of rotation (ICR) in the lumbar motion segmentin the neutral posture is typically located slightly posterior of thecenter of the intervertebral disc in a normal spine. Although the ICRshifts in an anterior and superior direction during spinal flexion andin a posterior direction during extension, the ICR typically remainssituated within the disc or within the upper aspect of the inferiorvertebra. Greater variations in the location of the ICR during movementare known to appear in degenerated spines. This occurrence of the ICRoutside normal physiological limits has been associated with spinalpathology.

In order to preserve the vertebral center of rotation at its naturallocation, embodiments of the interspinous spacer devices project thecenter of rotation to a location that is anterior of the device'simplanted location. This anterior projection of the center of rotationis accomplished by configuring the geometry of the device such that therotation does not occur along an internal axis. The concept by whichembodiments of the interspinous spacer devices disclosed herein projectthe center of rotation in an anterior direction is illustrated in FIG.2.

FIGS. 2A and 2B illustrate a round ball that is free to roll on either aconcave or a convex surface, respectively. As shown in FIG. 2A, a roundball that rolls on a concave surface will rotate around a center ofrotation in the geometric center of the ball. The cup-like concavesurface cradles the ball and projects the axis of rotation back into therolling ball. However, if the same ball rolls on a convex surface asshown in FIG. 2B, the center of rotation is projected downward and awayfrom the ball.

Embodiments of the interspinous spacer devices incorporate this conceptof center of rotation projection as a basis for the device's design. Byutilizing curved surfaces with a specific radius, the device may beconfigured such that the center of rotation of the stabilized spinalsegment is projected back into the anterior portion of the spine.Embodiments of the devices make use of a sliding motion for devicemovement rather than the conventional rotation of internal hingedelements typical of existing interspinous spacer devices. The anteriordistance that the center of rotation is projected from embodiments ofthe device may be controlled by altering the radius of the curvedsurfaces inside of the device.

b. Interspinous Spacer Devices

An embodiment of an interspinous spacer device 100 is shown fittedbetween two spinous processes of a spine segment model in FIG. 3. Theinterspinous spacer device 100 may be affixed to the spinous processesusing existing surgical fasteners suitable for orthopedic surgicalapplications including but not limited to pin connectors. Theinterspinous spacer device 100 may maintain a minimum spacing duringextension in order to increase the neuroforaminal height relative to thedegraded segment, and may also constrain the center of rotation of thesegment to be coincident with the center of rotation of an undegradedsegment during other movements, including but not limited to flexion,axial twisting, and lateral bending.

The interspinous spacer device 100 overcomes many of the limitations ofexisting interspinous spacer devices. Because the interspinous spacerdevice 100 includes only two moving parts, the likelihood of failure maybe substantially lower. Further, the close proximity of the interspinousspacer device 100 to the spinal column's natural line of loading mayresult in lower stresses on the interspinous spacer device 100 and thespinal processes to which the device 100 are attached, further reducingthe likelihood of failure of the device 100 or fracture of the spinalprocesses to which the device 100 is attached.

Referring to FIG. 4, the interspinous spacer device 100 comprises anouter base 300 that attaches to the caudal spinous process and an uppersliding rod 200 that attaches to the cranial spinous process. Thesliding rod 200 is free to slide relative to the base 300 through adistance limited by the dimensions of the base 300. All sliding surfacesof the interspinous spacer device 100 may possess a curvature, therebyconstraining the movement of the sliding rod 200 to an arcuate movement,resulting in the projection of the center of rotation of the movingsegment in an anterior direction during flexion, as illustrated in FIG.5.

i. Sliding Rod

Referring back to FIG. 4, the sliding rod 200 may include an enlargedretaining plate 202 situated on the lower end 216 of the sliding rod200. The retaining plate 202 prevents the sliding rod 200 fromseparating away from the base 300 during maximum separation of thevertebral processes to which the interspinous spacer device 100 isattached, such as may occur during maximum spinal flexion. By limitingthe maximum extension of the sliding rod 200 out of the base 300, theretaining plate 202 imposes a hard limit on the maximum rotation of thespinal segment during flexion.

The retaining plate 202 may have any non-circular cross-sectional shape,so long as the cross-sectional shape of the retaining plate 202 ismatched to the cross-sectional shape of the track, and is shaped suchthat the sliding rod 200 is constrained against freely rotating about anaxis coincident with the direction of sliding. The cross-sectionaldimensions of the retaining plate 202 are sized to be consistent withthe cross-sectional dimensions of the corresponding track 320 containedwithin the base 300. In an embodiment, the cross-sectional dimensionsand the thickness of the retaining plate 202 may be specified in orderto impart a desired range of motion to the interspinous spacer device100 in use. For example, if the cross-sectional dimensions of theretaining plate 202 are significantly smaller than the correspondingdimensions of the track, the range of movement of the spinal segmentstabilized by the interspinous spacer device 100 may be enhanced due tothe increased degree of movement or “play” of the sliding rod 200 withinthe track 320 perpendicular to the direction of sliding. Similarly, athicker retaining plate 202 may increase the contact area between theretaining plate 202 and the sides of the track 320, thereby reducing theamount of play of the sliding rod 200 within the track 320 and resultingin a reduced range of motion of the interspinous spacer device 100. Athicker retaining plate 202 may also reduce the maximum extension of thesliding rod 200 from the base 300.

Referring to FIG. 6, the retaining plate 202 includes an upper surface234, a lower surface 236, an anterior face 220, a posterior face 222,and two side faces 224 and 226. The side faces 224 and 226 providelateral support to the sliding rod 200 as it slides along the track 320.The anterior face 220 and posterior face 222 slide along the curvedportions of the track 320, imparting the arcuate sliding motion to thesliding rod 200. In an embodiment, the anterior face 220 and theposterior face 222 may be curved such the radius of curvature of eachface is matched to the radius of curvature of the corresponding surfacesof the track 320.

The sliding rod 200 further includes a column 204 attached at its lowercolumn end to the upper surface 234 of the retaining plate 202, and tothe cranial attachment fitting 210 at its opposite upper column end. Thecolumn 204 is a rigid structure capable of sustaining, withoutsignificant deformation, the loads imposed by the cranial vertebra towhich the sliding rod 200 is attached and by the caudal vertebra towhich the base 300 is attached. The column 204 may be anycross-sectional shape and dimension, so long as the column 204 fitswithin the track 320 without interfering with the sliding movement ofthe sliding rod 200 and the column 204 is able to slide smoothly throughthe aperture 312 of the base (see FIG. 4). In an embodiment, thecross-sectional shape of the column 204 is matched to thecross-sectional shape of the aperture 312 and the cross-sectionaldimensions of the column 204 are slightly smaller than the correspondingcross-sectional dimensions of the aperture 312. In another embodiment,the cross-sectional shape of the column 204 is matched to thecross-sectional shape of the track 320 in order to maintain all surfacesof the column 204 at a fixed distance away from the surfaces of thetrack 320.

Referring to FIG. 7, the anterior column surface 230 and posteriorcolumn surface 232 may be any shape, so long as these surfaces do notinterfere with the sliding movements of the sliding rod 200 within thebase 300. In an embodiment, the anterior column surface 230 andposterior column surface 232 may have a radius of curvature that ismatched to the corresponding surfaces of the track 320 within the base300.

Referring back to FIG. 6, the cranial attachment fitting 210 is attachedto the upper column end opposite to the retaining plate 202. The cranialattachment fitting 210 is shaped to fit around a vertebral processcranial to a degraded spinal segment. The cranial attachment fitting 210includes a lower plate 218 and two lateral plates 206 and 208 that areattached to the lateral edges of the lower plate 218 such that theyproject upward from the lower plate. The lateral plates 206 and 208 alsocontain two cranial fastener holes 212 and 214.

A spinal process cranial to a degraded spinal segment may be situatedwithin the groove formed by the lateral side plates 206 and 208 and thelower plate 218, and fasteners may be inserted through the cranialfastener holes 212 and 218 into the spinal process in order to fastenthe cranial attachment fitting 210 to the spinal process. Any knownfastener may be used to attach the cranial attachment fitting 210 to thespinal process, including but not limited to surgical-grade pins,screws, bolts, and any combination thereof.

In an embodiment, any or all curved sliding surfaces of the sliding rod200 may have a radius of curvature of about 50 mm in order to situatethe center of rotation of the stabilized spinal segment anteriorly intothe intervertebral disc of that segment. Non-limiting examples of curvedsliding surfaces of the sliding rod 200 include the anterior face 220and posterior face 222 of the retaining plate 202, the anterior columnsurface 230 and the posterior column surface 232. In another embodiment,the thickness of the retaining plate 202 may be about 3.5 mm.

ii. Base

Referring to FIG. 8, the base 300 includes a body 314 containing thetrack 314 (not shown) within which the sliding rod 200 slides. Caudalattachment plates 302 and 304 project downward from opposite edges ofthe body 314, forming a caudal attachment fitting 322. A spinal processcaudal to the segment to be stabilized may be situated within the grooveformed by the caudal attachment plates 302 and 304 and the lower surface318 of the body 314. Fasteners may be inserted through the caudalfastener holes 316 and 318 into the caudal spinal process in order tofasten the caudal attachment fitting 322 to the caudal spinal process.Any known fastener may be used to attach the caudal attachment fitting322 to the caudal spinal process, including but not limited tosurgical-grade pins, screws, bolts, and any combination thereof.

The body 314 of the base 300 contains an internal track (not shown) thatopens to an aperture 312 on the upper surface 310 of the base 300. Theaperture 312 may have any non-circular cross-sectional shape anddimension, so long as the aperture 312 allows the column 204 of theslider rod 200 to slide in and out of the base 300 without significantfriction, resistance, locking, sticking, or seizing. Further, thecross-sectional shape and dimension of the aperture 312 may besignificantly smaller than the corresponding cross-sectional dimensionsof the retaining plate 202 so that the retaining plate cannot passthrough the aperture 312, thereby constraining the full extension of thesliding rod 200 from the base 300.

Referring to FIG. 9, the base 300 contains an internal curved track 320surrounded by the outer surface of the body 314. The curved track 314includes a curved anterior track wall 324 and a curved posterior trackwall 326, as well as two flat lateral track walls. The anterior face 220of the retaining plate 202 may slide along the curved anterior trackwall 324, the posterior face 222 of the retaining plate 202 may slidealong the curved posterior track wall 326, and two side faces 224 and226 of the retaining plate 202 may slide along the two lateral trackwalls.

The shape and dimensions of the curved track 320 are critical aspects ofthe design of the interspinous spacer device 100. The height of thetrack 320 limits the range of rotational movement of the stabilizedspinal segment to within a physiological range corresponding to themovements of normal healthy vertebrae. The curvature of the track 320projects the center of rotation of the stabilized vertebra to a locationconsistent with the center of rotation of corresponding normal, healthyvertebrae. In one aspect, the radius of curvature of the track 320 isessentially equal to the distance between the anterior track surface 324and the center of rotation of the stabilized vertebra.

In one embodiment, the radius of curvature of the anterior track wall324 and a posterior track wall 326 are about 50 mm in order to situatethe center of rotation of the stabilized spinal segment anteriorly intothe intervertebral disc of that segment. In another embodiment, theheight of the curved track 320 is about 7.0 mm and the thickness of theretaining plate is about 3.5 mm. In this embodiment, the resultinginterspinous spacer device 100 has a range of motion that includes afully extended position of the sliding rod 200 from the base 300 ofabout 3.5 mm relative to fully retracted position, corresponding to amaximum of about 8.0° of rotation of the supported spinal segment duringspinal flexion.

c. Applications of Interspinous Spacer Devices

Embodiments of the interspinous spacer devices 100 may be used for thetreatment of high grade spinal disorders such as spinal stenosis as wellas facet arthrosis. The devices 100 may be used to stabilize spinalsegments at any location along the spine, including but not limited tothe cervical, thoracic, lumbar, and any combination thereof.

The sliding rod 200 of the interspinous spacer device 100 is free toslide inside of the base 300 within a limited range of motion asdescribed above to allow flexion-extension movement. In addition, theinterspinous spacer device 100 may be attached to the spinal processessituated cranial and caudal to the degraded spinal segment usingfasteners such as surgical pins that may further provide motion within alimited range in lateral bending and axial twisting.

In one embodiment, the interspinous spacer device 100 may be attached tospinous processes of vertebrae that are situated cranial or caudalrelative to a degraded motion segment, as shown in FIG. 3. The cranialattachment fitting 210 may be situated around the lower portion of aspinous process of the vertebra situated cranial to the degraded motionsegment. One or more surgical fasteners may be inserted through cranialfastener holes 212 and 218 to secure the cranial attachment fitting 210to the spinous process.

Similarly, the caudal attachment fitting 322 may be attached to aspinous process of a vertebra situated caudal to the degraded motionsegment. The caudal attachment fitting 322 may be situated around theupper portion of a spinous process of the vertebra situated cranial tothe degraded motion segment. One or more surgical fasteners may beinserted through caudal fastener holes 316 and 318 to secure the caudalattachment fitting 210 to the spinous process.

EXAMPLES

The following examples illustrate various embodiments of the invention.

Example 1 Theoretical Assessment of the Range of Motion of a StabilizedVertebral Segment Using Finite Element Modeling

Computational finite element modeling of the interspinous spacer deviceusing the analysis of a finite element model was performed in order toassess the interspinous spacer device's range of motion.

The base and sliding rod elements of the interspinous spacer device werecreated in Pro/ENGINEER (Version 5.0, PTC, Needham, Mass.) and importedinto ABAQUS (Version 6.9, Simulia, Providence, R.I.) for finite elementanalysis. The sliding rod and base were meshed with 48,436 and 32,710eight-node hexahedral brick elements respectively. Ti6Al4V materialproperties were assigned to each component.

A preliminary analysis was performed in order to determine the locationof the device's center of rotation. An encastre boundary condition wasimposed on the lower face of the base via a kinematic couplingconstraint while a superiorly directed tensile force of 150N was appliedto the upper surface of the sliding rod. A reference node situated 50 mmanterior of the device's sliding surfaces was kinematically coupled tothe sliding bar in order to track the translation of the center ofrotation through the full range of motion of the device. The results ofthis analysis determined that the center of rotation was situated about50 mm anterior of the device's sliding surfaces. This anterior locationplaced the center of rotation directly within the center of theintervertebral disc associated with the stabilized segment, asillustrated in FIG. 10.

The model of the interspinous spacer device was implanted on the L1-L5lumbar spine model to further assess the range of movement of thestabilized spinal segment and adjacent segments for various conditionsof the spinal segment, as illustrated in FIG. 11. An encastre boundarycondition was imposed on the inferior L5 endplate while 7.5 Nmflexion/extension moments were applied to the superior endplate of L1. Afriction coefficient of 0.3 was specified for the device's slidingsurfaces.

The range of motion was assessed for a healthy spine model, an injuredspine model with full nucleus pulposus removal at the L2-L3 level, andan injured spine model with the interspinous spacer device implanted.Implantation was modeled between the spinous processes of L2 and L3. Theupper and lower components of the device were fastened to the spinousprocesses via tie constraints.

The range of motion as a function of applied flexion/extension momentdetermined by the finite element models are summarized in FIG. 12. Therotation of the segment models as well as the total rotation of allsegments in response to a 7.5 Nm flexion/extension moment are summarizedin FIG. 13. The range of motion for the L1-L2 motion segment situatedsuperior to the injured segment was unaltered by the injury or spacerimplantation (see FIG. 12A and FIG. 13). Loading behavior at theimplanted level (L2-L3) was identical to the intact condition (see FIG.13). Total rotation at the injured level rose by 4.5° as a result of thenucleus removal (see FIG. 13). Implantation at the injured levelrestored total rotation to within 0.5° of the intact case (see FIG. 13),but shifted the loading curve by reducing extension rotation andincreasing flexion rotation (see FIG. 12B). Rotation at the L3-L4 andL4-L5 levels was not greatly altered due to injury or implantinstallation.

The results of this experiment confirmed that the center of rotation ofan injured motion segment that was stabilized with the interspinousspacer device was situated in a region consistent with the uninjuredsegment's center of rotation. In addition, injury to a motion segmentresulted in a higher range of motion relative to the uninjured state.Stabilization of the injured segment using the interspinous spacerdevice restored the range of motion of the injured segment to uninjuredlevels, with no discernable effect on motion segments situated cranialor caudal to the injured segment.

Example 2 Assessment of the Stress Distribution of an InterspinousSpacer Device

To assess the distribution of stresses acting on an interspinous spacerdevice during stabilization of a spinal segment, the followingexperiment was conducted. The finite element model of the injured spinalsegment stabilized using an interspinous spacer device similar to themodel described in Example 1 was used to determine the stresses actingon the device during applied flexion/extension moments of up to 7.5 Nm.The results of these experiments are presented in FIG. 14 and FIG. 15for the sliding rod and base of the interspinous spacer device,respectively.

Peak stresses during spinal flexion occurred at the base of the slidingrod, as illustrated in FIG. 14. The maximum von Mises stresses observedin this region were 119 MPa. The peak von Mises stresses in the upperregion of the column of the sliding rod near the cranial attachmentfitting reached 53 MPa. Peak stresses on the base occurred in the regionof the anterior edge of the aperture as illustrated in FIG. 15. Stresseson the base reached a maximum of 27.2 Mpa, which was significantly lowerthan the stresses observed for the sliding rod. The stress levels on thebase fell below the fatigue limit of titanium, indicating that failureof the implant was not likely during loading associated with spinalflexion.

The location of peak stresses on the upper sliding rod occurred at thesame location in extension as for flexion. The maximum von Mises stressduring extension at the corner of the sliding rod indicated in FIG. 14was 488 MPa. The stress at the adjacent location on the base piece was164 MPa. These stresses were deemed unacceptably high for the device tomaintain structural integrity over the lifetime of a patient receivingtreatment using the device.

The results of these experiments determined that the interspinous spacerdevice may sustain unacceptably high stress levels during simulatedspinal extension, indicating a risk of device failure with the designtested. Modifications to the geometry device to increase the size andamount of material used to produce the interspinous spacer device mayreduce the stress levels to within acceptable levels.

Example 3 Finite Element Assessment of the Range of Motion of aVertebral Segment Stabilized Using a Revised Interspinous Spacer DeviceDesign

To assess the range of motion and center of rotation of a vertebralsegment stabilized using a modified interspinous spacer device, thefollowing experiments were conducted. A finite element model of aninterspinous spacer device with a modified design relative to the deviceanalyzed in Example 1 was implanted on the spinal segment model asdescribed in Example 1. The finite element model used in theseexperiments is illustrated in FIG. 16. The center of rotation and rangeof motion of the modified interspinous spacer device during spinalflexion and extension was assessed using methods and conditions similarto those described in Example 1.

The center of rotation location during flexion and extension of thespinal segment stabilized using the modified interspinous spacer devicewas comparable to the center of rotation locations observed for theprevious design described in Example 1.

FIGS. 17A-D summarize the range of motion as a function of the appliedflexion/extension moment for four levels of a spinal segment stabilizedusing the modified interspinous spacer. The range of motion of thestabilized spinal segment was closer to the range of motion of theintact segment than the range of motion for the previous interspinousspacer shown in FIG. 12. The loading curve for the implanted L2-L3 levelshown in FIG. 17B indicated restoration of rotation in flexion andextension that closely corresponded to the intact segment.

The maximum intervertebral rotations of the four different levels of thespinal segment as well as the total rotation of all segments for theintact, injured, and implanted spine models are summarized in FIG. 18.Intervertebral rotations of the implanted spine returned to within 0.5°of the intact spine for all corresponding spinal segment levels afterinstallation of the modified interspinous spacer device, and the totallumbar rotation returned to within 1.0° with the addition of the device.

The results of this experiment verified that the stabilization of aninjured spinal segment using the modified interspinous spacer deviceresulted in intersegmental rotation angles that were consistent with theintact spine model.

Example 4 Assessment of the Stress Distribution of a ModifiedInterspinous Spacer Design

To assess the distribution of stresses acting on a modified interspinousspacer device similar to the device described in Example 3 duringstabilization of a spinal segment, the following experiment wasconducted. The finite element model of the injured spinal segmentstabilized using the modified interspinous spacer device described inExample 3 was used to determine the stresses acting on the device duringapplied flexion/extension moments of up to 7.5 Nm using methods similarto those described in Example 2.

A major goal of the modified interspinous spacer design was to reducethe stresses experienced by the implant in extension, these experimentsfocused mainly on this loading condition. Further, because the highestpeak stresses were observed in Example 2 on the sliding rod of theinterspinous spacer device, the peak stresses acting on the sliding rodwere of particular interest.

The results of the finite element analysis indicated that the peak vonMises stress on the base of the modified interspinous spacer device was17 MPa. The peak von Mises stress observed on the sliding rod was 67.1Mpa, and was situated in a similar location to the peak stress observedin Example 2 (see FIG. 19). These peak von Mises results fell well belowthe fatigue limit of the Ti6Al4V material from which the modifiedinterspinous device was constructed.

The results of this experiment indicated that the modified design of theinterspinous spacer device resulted in significantly lower peak stressescompared to the peak stresses observed for the previous design inExample 2. These results support the conclusion that the modified devicewould successfully withstand fatigue testing, and posed little risk offailure over a lifetime of use to stabilize a degraded spinal segment ofa patient.

It should be understood from the foregoing that, while particularembodiments have been illustrated and described, various modificationscan be made thereto without departing from the spirit and scope of theinvention as will be apparent to those skilled in the art. Such changesand modifications are within the scope and teachings of this inventionas defined in the claims appended hereto.

What is claimed is:
 1. An implantable interspinous spacer forimplantation at an implantation site between a cranial spinous processand a caudal spinous process, each spinous process extending from arespective vertebral body of a spinal column of vertebral bodies, thespinal column including an anterior-posterior center that is anteriorthe implantation site, the implantable interspinous spacer comprising: afirst element comprising: a first anchor structure configured to anchorto a first of the cranial spinous process or the caudal spinal process;and a body operably coupled to the first anchor structure, the bodycomprising an outer surface and an internal track surrounded by theouter surface, the internal track comprising a first sliding surfaceextending cranial-caudal when the implantable interspinous spacer isimplanted at the implantation site; and a second element comprising: asecond anchor structure configured to anchor to a second of the cranialspinous process or the caudal spinous process; and a retaining plateoperably coupled to the second anchor structure and configured to beretained within, and prevent withdrawal from, the internal track of thebody, the retaining plate comprising a second sliding surface configuredto slidingly displace along the first sliding surface when theimplantable interspinous spacer is implanted at the implantation site;wherein the first sliding surface includes a radius of curvature that isgenerally equal to a distance extending between the anterior-posteriorcenter and the implantation site.
 2. The implantable interspinous spacerof claim 1, wherein a curvature direction of the first sliding surfaceis such that, when the implantable interspinous spacer is implanted atthe implantation site, a center of rotation associated with theimplantation of the implantable interspinous spacer is created at theanterior-posterior center.
 3. The implantable interspinous spacer ofclaim 2, wherein the implantable interspinous spacer is configured suchthat an extension-retraction range of movement resulting from theimplantation of the implantable interspinous spacer at the implantationsite corresponds to a rotation about the center of rotation ofapproximately eight degrees.
 4. The implantable interspinous spacer ofclaim 1, wherein the radius of curvature is approximately 50 mm.
 5. Theimplantable interspinous spacer of claim 1, wherein the first slidingsurface is at least part of an anterior sliding surface.
 6. Theimplantable interspinous spacer of claim 5, wherein the internal trackfurther comprises another sliding surface posterior the first slidingsurface and also extending cranial-caudal when the implantableinterspinous spacer is implanted at the implantation site, and theretaining plate further comprises another sliding surface posterior thesecond sliding surface, the other sliding surface of the retaining platebeing configured to slidingly displace along the other sliding surfaceof the internal track when the implantable interspinous spacer isimplanted at the implantation site.
 7. The implantable interspinousspacer of claim 1, wherein the first element is a caudal element and thefirst anchor structure is configured to anchor to the caudal spinousprocess, and the second element is a cranial element and the secondanchor structure is configured to anchor to the cranial spinous process.8. The implantable interspinous spacer of claim 1, wherein at least oneof the first anchor structure or second anchor structure is configuredto receive and wrap around the cranial spinous process or the caudalspinous process.
 9. The implantable interspinous spacer of claim 1,wherein at least one of the first anchor structure or second anchorstructure comprises opposed side members joined together by, andextending away from, another member, wherein the associated cranialspinous process or the caudal spinous process is received between theopposed side members in the course of being anchored to the at least oneof the first anchor structure or the second anchor structure.
 10. Theimplantable interspinous spacer of claim 1, wherein a cranial-caudaldisplacement distance of the second sliding surface relative to thefirst sliding surface is limited to approximately 3.5 mm.
 11. Theimplantable interspinous spacer of claim 1, wherein a cranial-caudallength of the first sliding surface is approximately 7 mm.
 12. Theimplantable interspinous spacer of claim 11, wherein a cranial-caudallength of the second sliding surface is approximately 3.5 mm.
 13. Theimplantable interspinous spacer of claim 1, wherein the internal trackcomprises a curved track arrangement that includes the first slidingsurface.
 14. The implantable interspinous spacer of claim 1, wherein thefirst sliding surface and the first anchor structure are of a unitaryconstruction relative to each other in forming the first element. 15.The implantable interspinous spacer of claim 1, wherein a cranial-caudallength of the first sliding surface of the internal track and acranial-caudal length of the second sliding surface equal to a thicknessof the retaining plate determine a maximum extension-retraction range ofmovement of the implantable interspinous spacer when implanted at theimplantation site.
 16. A method of treating a spinal condition employingan implantable interspinous spacer for implantation at an implantationsite between a cranial spinous process and a caudal spinous process,each spinous process extending from a respective vertebral body of aspinal column of vertebral bodies, the spinal column including ananterior-posterior center that is anterior the implantation site, themethod comprising: anchoring a first anchor structure of the implantableinterspinous spacer to a first of the cranial spinous process or thecaudal spinal process; and anchoring a second anchor structure of theimplantable interspinous spacer to a second of the cranial spinousprocess or the caudal spinal process; wherein the implantableinterspinous spacer comprises: a first element comprising: the firstanchor structure; and a body operably coupled to the first anchorstructure, the body comprising an outer surface and an internal tracksurrounded by the outer surface, the internal track comprising a firstsliding surface extending cranial-caudal when the implantableinterspinous spacer is implanted at the implantation site; and a secondelement comprising: the second anchor structure; and a retaining plateoperably coupled to the second anchor structure and configured to beretained within, and prevent withdrawal from, the internal track of thebody, the retaining plate comprising a second sliding surface configuredto slidingly displace along the first sliding surface when theimplantable interspinous spacer is implanted at the implantation site;wherein the first sliding surface includes a radius of curvature that isgenerally equal to a distance extending between the anterior-posteriorcenter and the implantation site.
 17. The method of claim 16, whereinthe first element is a caudal element and the first anchor structure isconfigured to anchor to the caudal spinous process, and the secondelement is a cranial element and the second anchor structure isconfigured to anchor to the cranial spinous process.
 18. The method ofclaim 16, wherein the first anchor structure is configured to receiveand wrap around the caudal spinous process, and the second anchorstructure is configured to receive and wrap around the cranial spinousprocess.
 19. The method of claim 16, wherein at least one of the firstanchor structure or second anchor structure comprises opposed sidemembers joined together by, and extending away from, another member,wherein the associated cranial spinous process or the caudal spinousprocess is received between the opposed side members in the course ofbeing anchored to the at least one of the first anchor structure or thesecond anchor structure.
 20. The method of claim 16, wherein, in beingimplanted at the implantation site, the first anchor and the secondanchor do not anchor to the vertebral body of the respective spinousprocess.